Optical routing devices such as optical switches and, in particular wavelength selective switches (WSS), are used in optical communications and optical measurement applications. Conventional optical routing devices, such as those disclosed in U.S. Pat. No. 6,097,859 issued Aug. 1, 2000 to Solgaard et al; U.S. Pat. No. 6,498,872 issued Dec. 24, 2002 to Bouevitch et al; U.S. Pat. No. 6,707,959 issued Mar. 16, 2004 to Ducellier et al; U.S. Pat. No. 6,810,169 issued Oct. 26, 2004 to Bouevitch, and U. S. Pat. Publication No. 2007/0242953 published Oct. 18, 2007 to Keyworth et al, which are incorporated herein by reference, separate a multiplexed optical beam into constituent wavelengths with a dispersive element, and then direct individual wavelengths or groups of wavelengths, which may or may not have been modified, back through the device to a desired output port.
In a typical WSS the input and output optical ports are faces of optical fibers that are arranged in an array wherein fibers are equally spaced and held in position in a fiber array unit (FAU). The front end of the device may further include a polarization diversity unit for ensuring the beam of light routed within the device has a single state of polarization. The back end of the device typically includes a switching engine, also referred to herein as a beam director, wherein individually controllable devices are used to redirect selected wavelengths back to one of several output ports.
An optical beam emerging from the FAU is transformed by the WSS optics into multiple beams according to the wavelength, and made to converge at the switching engine. The WSS optics is effective in mapping the optical fiber position in the FAU to a beam angle at the switching engine. The switching engine operates by imparting a controllable tilt to an incoming beam, to redirect it to an output port. Possible switching engine technologies include tiltable micro-mirrors of a micro electromechanical switch (MEMS) and liquid crystal on silicon (LCoS) phase arrays. The switching engine is typically voltage controlled, with the amount of tilt generally increases with the applied voltage.
In addition to switching between ports, it may be desirable for the switching engine to attenuate optical signals. This variable attenuation may be accomplished by tilting the beam slightly away from an angle corresponding to an optimal optical alignment with a selected output fiber port, such that the beam impinges thereupon with some controllable offset and suffers a coupling loss into the output fiber. Optical switches wherein signals are variably attenuated by tilting micro-mirrors away from the optimal optical alignment with a target optical port are disclosed for example in U.S. Pat. No. 6,798,941 to Smith et al and U.S. Pat. No. 7,142,744 to Walter et al, which are incorporated herein by reference.
Unfortunately, when an output optical beam is offset from its optimal alignment with a target output optical port in a multiport routing device, the beam may come into a partial alignment with an adjacent optical port, increasing the amount of light leaking into the non-selected adjacent port, resulting in an undesirable optical crosstalk between adjacent optical ports in the array.